One-piece solid golf ball

ABSTRACT

The present invention provides a one-piece solid golf ball having excellent rebound characteristics and good shot feel, while maintaining excellent processability and durability. The present invention relates to a one-piece solid golf ball formed from a rubber composition comprising a mixture consisting of polybutadiene (a) synthesized using nickel-containing catalyst and polybutadiene (b) synthesized using cobalt-containing catalyst and hydroquinone or derivatives thereof as a vulcanization stabilizer, wherein a Mooney viscosity and a weight ratio of the polybutadienes (a) and (b), an amount of the vulcanization stabilizer, a center hardness (the minimum hardness in the golf ball) and surface hardness of the golf ball, and a difference between the maximum hardness and minimum hardness in the golf ball are adjusted to a specified range.

FIELD OF THE INVENTION

[0001] The present invention relates to a one-piece wound golf ball. More particularly, it relates to a one-piece wound golf ball having excellent rebound characteristics and good shot feel, while maintaining excellent processability and durability.

BACKGROUND OF THE INVENTION

[0002] Golf balls are typically classified into two types, that is, golf balls for round games and golf balls for driving ranges. It is difficult to say that performance, which is required for these two types of golf balls, is the same with each other. The performance required for the golf ball for round games is good shot feel, excellent flight performance and the like. If the golf balls for round games are used as a golf ball for driving ranges, good shot feel and excellent flight performance are maintained, but the durability is very poor as a golf ball for driving ranges, which is repeatedly hit. Therefore, at driving ranges, the durability is regarded as more important than the shot feel and flight performance, and golf balls having much better durability than that of the golf balls for round games is used.

[0003] In the golf balls for driving ranges, there are golf balls having low rebound characteristics, low trajectory and high trajectory, golf balls for using on the water and the like, depending on the extent of the driving ranges, the height of net and the like. There are various golf balls, such as one-piece golf ball, two-piece golf ball and the like, depending on the structure of the golf ball. At present, one-piece solid golf ball is mainly used as golf balls for driving ranges.

[0004] Recently, since performance as good as golf balls for round games, such as shot feel, has been required for golf balls for driving ranges, good shot feel and excellent flight performance have been required for the one-piece golf ball having excellent durability. However, when softening the golf ball in order to improve the shot feel or improving the flight performance, the durability is degraded. It is very difficult to improve shot feel and flight performance, while maintaining excellent durability.

[0005] A rubber composition comprising high-cis polybutadiene, methacrylic acid (or metal salt thereof) as a co-crosslinking agent and organic peroxide has been generally used as a general formulation of one-piece golf ball. When zinc acrylate, which is used as a co-crosslinking agent of rubber composition for a core of two-piece golf ball, is used for one-piece golf ball, rebound characteristics are good, but durability is very poor. Therefore it is not generally used for one-piece golf ball.

[0006] A one-piece solid golf ball, which is formed by using polybutadiene rubber having high Mooney viscosity, and have even hardness distribution, is proposed (for example, Japanese Patent Kokai Publication No. 177973/1990, Japanese Patent No. 2644226 and the like). In Japanese Patent Kokai Publication No. 177973/1990, a one-piece solid golf ball, which is obtained by vulcanizing and press-molding a rubber composition comprising polybutadiene having high Mooney viscosity, and have even hardness distribution so that difference between the maximum hardness and minimum hardness in the golf ball is not less than 15, is described. However, in the golf ball, since the polybutadiene having high Mooney viscosity is used, molecular weight of the polybutadiene rubber is high, and the processability is poor. In addition, the rebound characteristics are improved, but the durability is degraded, and the shot feel is poor because of even hardness distribution.

[0007] In Japanese Patent No. 2644226, a solid golf ball formed by using a rubber composition comprising polybutadiene, of which Mooney viscosity (45 to 90 ML₁₊₄ (100° C.)) and a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) are adjusted to specified ranges is described. However, as described in the above golf ball, since the polybutadiene having high Mooney viscosity is used, molecular weight of the polybutadiene rubber is high, and the processability is poor. In addition, the rebound characteristics are improved, but the durability is degraded.

OBJECTS OF THE INVENTION

[0008] A main object of the present invention is to provide a one-piece wound golf ball having excellent rebound characteristics and good shot feel, while maintaining excellent processability and durability.

[0009] According to the present invention, the object described above has been accomplished by using a rubber composition comprising a mixture consisting of polybutadiene (a) synthesized using nickel-containing catalyst and polybutadiene (b) synthesized using cobalt-containing catalyst and hydroquinone or derivatives thereof as a vulcanization stabilizer, and adjusting a Mooney viscosity and a weight ratio of the polybutadienes (a) and (b), an amount of the vulcanization stabilizer, a center hardness (the minimum hardness in the golf ball) and surface hardness of the golf ball, and a difference between the maximum hardness and minimum hardness in the golf ball to a specified range, thereby providing a one-piece wound golf ball having excellent rebound characteristics and good shot feel, while maintaining excellent processability and durability.

SUMMARY OF THE INVENTION

[0010] The present invention provides a one-piece wound golf ball formed from a rubber composition comprising a base rubber, a co-crosslinking agent, an organic peroxide, an inorganic filler and a vulcanization stabilizer, and the base rubber comprises a polybutadiene mixture consisting of

[0011] (i) a polybutadiene (a) containing a cis-1,4 bond of not less than 96% and having a Mooney viscosity of 50 to 85 ML₁₊₄(100° C.) and a weight average molecular weight (Mw) of 70×10⁴ to 120×10⁴, synthesized using nickel-containing catalyst, and

[0012] (ii) a polybutadiene (b) containing a cis-1,4 bond of not less than 96% and having a Mooney viscosity of 30 to 50 ML₁₊₄(100° C.) and a weight average molecular weight (Mw) of not less than 55×10⁴ and less than 70×10⁴, synthesized using cobalt-containing catalyst, a weight ratio (a)/(b) being 95/5 to 70/30, wherein

[0013] the vulcanization stabilizer is selected from the group consisting of hydroquinone and derivatives thereof, and an amount of the vulcanization stabilizer is 0.05 to 20 parts by weight, based on 100 parts by weight of the base rubber, and

[0014] the golf ball has a center hardness in JIS-C hardness of 55 to 68 and a surface hardness in JIS-C hardness of 75 to 90, the center hardness is the minimum hardness in the golf ball, and a difference between the maximum hardness and minimum hardness in the golf ball is within the range of 16 to 25.

[0015] In order to put the present invention into a more suitable practical application, the vulcanization stabilizer is preferably 2,5-di-t-butylhydroquinone, and the polybutadiene (b) synthesized using cobalt-containing catalyst preferably has a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 3.0 to 5.0.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The one-piece solid golf ball of the present invention is formed from integrally molded article obtained by press-molding and vulcanizing a rubber composition. The rubber composition comprises a base rubber, a co-crosslinking agent, an organic peroxide, an inorganic filler, a vulcanization stabilizer and optionally an antioxidant, and the like.

[0017] In the golf ball of the present invention, it is required for the base rubber to comprise a polybutadiene mixture consisting of

[0018] (i) a polybutadiene (a) containing a cis-1,4 bond of not less than 96% and having a Mooney viscosity of 50 to 85 ML₁₊₄(100° C.) and a weight average molecular weight (Mw) of 70×10⁴ to 120×10⁴, synthesized using nickel-containing catalyst, and

[0019] (ii) a polybutadiene (b) containing a cis-1,4 bond of not less than 96% and having a Mooney viscosity of 30 to 50 ML₁₊₄(100° C.) and a weight average molecular weight (Mw) of not less than 55×10⁴ and less than 70×10⁴, synthesized using cobalt-containing catalyst, in a weight ratio (a)/(b) of 95/5 to 70/30.

[0020] Nickel-containing catalysts used to synthesize the polybutadiene (a) in the present invention are, for example, one-component catalysts such as nickel on diatomaceous earth as a carrier, two-component catalysts such as Raney nickel/titanium tetrachloride, and three-component catalysts such as nickel compound/organometal/trifluoroborate etherate. Examples of nickel compounds include reduced nickel on carrier, Raney nickel, nickel oxide, nickel carboxylate, organic nickel complex salts and the like. Examples of the organometals include trialkyl aluminums such as triethyl aluminum, tri-n-propyl aluminum, triisobutyl aluminum, and tri-n-hexyl aluminum; alkyl lithiums such as n-butyl lithium, s-butyl lithium, t-butyl lithium and 1,4-butane dilithium; dialkyl zincs such as diethyl zinc, dibutyl zinc; and the like.

[0021] Polymerization of butadiene in the presence of these catalysts is generally carried out by continuously charging butadiene monomer into a reactor along with a conventional solvent and the catalyst such as nickel octanoate and triethyl aluminum, and controlling the reaction temperature in the range of 5 to 60° C. and the reaction pressure in the range of 1 to about 70 atmospheres, such that a product having desired properties such as Mooney viscosity may be obtained.

[0022] In the golf ball of the present invention, it is required for the polybutadiene (a) to contain a cis-1,4 bond of not less than 96%, and have a Mooney viscosity of 50 to 85 ML₁₊₄(100° C.), preferably 50 to 70 ML₁₊₄(100° C.), more preferably 55 to 65 ML₁₊₄(100° C.). When the content of the cis-1,4 bond in the polybutadiene (a) is less than 96%, the rebound characteristics of the resulting golf ball are not sufficiently obtained. When the Mooney viscosity of the polybutadiene (a) is lower than 50 ML₁₊₄(100° C.), the processability when mixing the rubber composition is good, but the rebound characteristics of the resulting golf ball are degraded. On the other hand, when the Mooney viscosity is higher than 85 ML₁₊₄(100° C.), the rebound characteristics of the resulting golf ball are sufficiently obtained, but the processability when mixing is degraded, which reduces productivity.

[0023] In the golf ball of the present invention, it is required for the polybutadiene (a) to have a weight average molecular weight (Mw) of 70×10⁴ to 120×10⁴, preferably 80×10⁴ to 110×10⁴, more preferably 80×10⁴ to 100×10⁴. When the Mw of the polybutadiene (a) is lower than 70×10⁴, the rebound characteristics of the resulting golf ball are degraded. On the other hand, when the Mw of the polybutadiene (a) is higher than 120×10⁴, the processability when mixing is degraded, which reduces productivity.

[0024] In the golf ball of the present invention, it is desired for the polybutadiene (a) to have a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 3.0 to 6.0, preferably 4.0 to 5.5. The ratio (Mw/Mn) represents molecular weight distribution. When the ratio (Mw/Mn) of the polybutadiene (a) is smaller than 3.0, the molecular weight distribution of the polybutadiene (a) is too narrow, and the compatibility with the polybutadiene (b) is degraded. On the other hand, when the ratio (Mw/Mn) of the polybutadiene (a) is larger than 6.0, the molecular weight distribution of the polybutadiene (a) is too broad, and the content of the low molecular weight component of the polybutadiene is large, which reduces the rebound characteristics.

[0025] In the golf ball of the present invention, it is desired for the polybutadiene (a) to have a number average molecular weight (Mn) of 10×10⁴ to 30×10⁴, preferably 15×10⁴ to 25×10⁴. When the Mn of the polybutadiene (a) is lower than 10×10⁴, the rebound characteristics of the resulting golf ball are not sufficiently obtained. On the other hand, when the Mn of the polybutadiene (a) is higher than 30×10⁴, the processability when mixing is degraded.

[0026] Examples of the polybutadiene (a), which will be shown by a trade name thereof, include “BR-18”, which is commercially available from JSR Co., Ltd., and the like.

[0027] The term “Mooney viscosity” as used herein refers to an indication of a viscosity which is measured using a Mooney viscometer as a kind of rotational plastometer. The Mooney viscosity is typically used for measuring a viscosity of a rubber composition in the field of rubber industry. The Mooney viscosity is determined by closely putting a rubber composition in a gap between a cylindrical dice and a rotor positioned at the center of the dice, and then measuring a torque occurring when rotating a rotor at a testing temperature of 100° C., for a preheating time of 1 minute, at the number of revolutions of 2 rpm, for the time of revolution of 4 minutes. The Mooney viscosity is expressed in ML₁₊₄(100° C.), wherein M represents a Mooney viscosity, L represents a large rotor (L type) as a shape of the rotor, (1+4) represents that a preheating time is 1 minute and a time of revolution of the rotor is 4 minutes, and 100° C. represents a testing temperature. The measurement is generally conducted according to JIS K 6300.

[0028] Cobalt-containing catalysts used to synthesize the polybutadiene (b) in the present invention are, for example, cobalt metal and cobalt compounds such as Raney cobalt, cobalt chloride, cobalt bromide, cobalt iodide, cobalt oxide, cobalt sulfate, cobalt carbonate, cobalt phosphate, cobalt phthalate, cobalt carbonyl, cobalt acetylacetonate, cobalt diethyldithiocarbamate, [(C₂H₅)₂NCS₂]₂Co, cobalt aniliniumnitrile, [(C₆H₅NH₃)₃Co (NO₂)], cobalt dinitrosyl chloride, and the like. In particular, combinations of these cobalt compounds with a dialkyl aluminum monochloride such as diethylaluminum monochloride and diisobutylaluminum monochloride; a trialkyl aluminum such as triethylaluminum, tri-n-propylaluminum, triisobutylaluminum and tri-n-hexylaluminum; an alkyl aluminum sesquichloride (Al₂R₃Cl₃) such as ethylaluminum sesquichloride; aluminum chloride; and the like are preferred catalysts for use in the preparation of polybutadiene of cis-1,4 bond type.

[0029] Polymerization of butadiene in the presence of the cobalt-containing catalysts is generally carried out by the similar process to that used with the nickel-containing catalysts.

[0030] In the golf ball of the present invention, it is required for the polybutadiene (b) to contain a cis-1,4 bond of not less than 96%, and have a Mooney viscosity of 30 to 50 ML₁₊₄(100° C.), preferably 33 to 43 ML₁₊₄(100° C.). When the content of the cis-1,4 bond in the polybutadiene (b) is less than 96%, the rebound characteristics of the resulting golf ball are not sufficiently obtained. When the Mooney viscosity of the polybutadiene (b) is lower than 30 ML₁₊₄(100° C.), the processability when mixing the rubber composition is good, but the rebound characteristics of the resulting golf ball are degraded. On the other hand, when the Mooney viscosity is higher than 50 ML₁₊₄(100° C.), the durability is degraded and the processability when mixing is greatly degraded.

[0031] In the golf ball of the present invention, it is required for the polybutadiene (b) to have a weight average molecular weight (Mw) of not less than 55×10⁴ and less than 70×10⁴, preferably 58×10⁴ to 65×10⁴. When the Mw of the polybutadiene (b) is lower than 55×10⁴, the rebound characteristics of the resulting golf ball are degraded. On the other hand, when the Mw of the polybutadiene (b) is within the range of not less than 70×10⁴, the durability is degraded.

[0032] In the golf ball of the present invention, it is desired for the polybutadiene (b) to have a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 3.0 to 5.0, preferably 3.5 to 4.5. The ratio (Mw/Mn) represents molecular weight distribution. When the ratio (Mw/Mn) of the polybutadiene (b) is smaller than 3.0, the molecular weight distribution of the polybutadiene (b) is too narrow, and the content of the low molecular weight component and high molecular weight component of the polybutadiene is small, which degrades the durability. On the other hand, when the ratio (Mw/Mn) of the polybutadiene (b) is larger than 5.0, the molecular weight distribution of the polybutadiene (b) is too broad, and the content of the low molecular weight component and high molecular weight component of the polybutadiene is large. Therefore, the compatibility is degraded, and the processability when mixing is degraded.

[0033] Polybutadiene synthesized using cobalt-containing catalyst generally tends to have broader molecular weight distribution than polybutadiene synthesized using nickel-containing catalyst, which varies depending on the polymerization condition such as concentration of monomer, concentration of catalyst, polymerization temperature, kind of solvent and the like. Therefore, in the present invention, a ratio of the low molecular weight component to the high molecular weight component of the polybutadiene mixture is controlled by adjusting the Mw/Mn of the polybutadiene (b) synthesized using cobalt-containing catalyst to the above specified range, and the processability when mixing and durability are improved.

[0034] In the golf ball of the present invention, it is desired for the polybutadiene (b) to have a number average molecular weight (Mn) of 10×10⁴ to 30×10⁴, preferably 10×10⁴ to 25×10⁴. When the Mn of the polybutadiene (b) is lower than 10×10⁴, the rebound characteristics of the resulting golf ball are not sufficiently obtained. On the other hand, when the Mn of the polybutadiene (b) is higher than 30×10⁴, the processability when mixing is degraded.

[0035] Examples of the polybutadiene (b), which will be shown by a trade name thereof, include “BR230”, “BR230”, which are commercially available from Ube Industries, Ltd., and the like.

[0036] In the golf ball of the present invention, it is required that a weight ratio (a)/(b) of polybutadiene (a) to polybutadiene (b) in the polybutadiene mixture be 95/5 to 70/30, preferably 95/5 to 80/20. When the amount of polybutadiene (a) is smaller than 70% by weight and the amount of polybutadiene (b) is larger than 30% by weight, based on the total weight of the polybutadiene mixture, the rebound characteristics of the resulting golf ball are degraded. On the other hand, when the amount of polybutadiene (b) is smaller than 5% by weight and the amount of polybutadiene (a) is larger than 95% by weight, based on the total weight of the polybutadiene mixture, the durability is degraded.

[0037] Generally, when the Mw of polybutadiene is high, the rebound characteristics tend to be improved, but the processability and durability tend to be degraded. On the other hand, when the Mw of polybutadiene is low, the processability tends to be improved, but the rebound characteristics tend to be degraded. When using the polybutadiene (b) alone, the processability is good, but the rebound characteristics are degraded. When using the polybutadiene (a) alone, the rebound characteristics are improved, but the processability is degraded, which reduces productivity. A one-piece solid golf ball, of which the rebound characteristics are improved, the processability is good and the durability is excellent, can be obtained by using the mixture of the polybutadiene (a) and polybutadiene (b) in the above weight ratio (a)/(b).

[0038] The “processability” as used herein is determined by evaluating the winding of the rubber composition on the mixing roll during mixing, and the surface roughening of plug when extruding the plug (unvulcanized molded article) used at molding. When the processability is poor, it is difficult to wind the rubber composition around the mixing roll, the plug has the surface roughening, the dispersibility of formulation materials is poor, and the performance of the resulting golf ball is uneven. In addition, the appearance is poor because of the surface roughening of plug, or the durability is degraded because the release agent gets into the surface roughening of plug.

[0039] The base rubber may be only the polybutadiene (a) and polybutadiene (b), but the polybutadienes may be optionally mixed with natural rubber, polyisoprene rubber, styrene-butadiene rubber, ethylene-propylene-diene rubber (EPDM), and the like. If used, it is desired for the total amount of the polybutadiene (a) and polybutadiene (b) to be not less than 80% by weight, preferably not less than 90% by weight, based on the total weight of the base rubber. When the total amount of the polybutadiene (a) and polybutadiene (b) is smaller than 80% by weight, the rebound characteristics are degraded and the processability is degraded.

[0040] Examples of the co-crosslinking agents include α,β-unsaturated carboxylic acids having 3 to 8 carbon atoms (e.g. acrylic acid, methacrylic acid, etc.) or a monovalent or divalent metal salt such as zinc or magnesium salt thereof; metal salt of α,β-unsaturated carboxylic acids, which is formed by reacting the α,β-unsaturated carboxylic acids with metal oxide such as zinc oxide during mixing rubber composition; polyfunctional acrylic esters such as trimethylolpropane triacrylate, polyfunctional methacrylic esters such as trimethylpropane trimethacrylate, and the like. The preferred co-crosslinking agent is metal salt of acrylic acid or methacrylic acid because it imparts excellent durability to the resulting golf ball. The amount of the co-crosslinking agent is from 19 to 29 parts by weight, preferably from 22 to 26 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the co-crosslinking agent is larger than 29 parts by weight, the resulting golf ball is too hard, and the shot feel is poor. On the other hand, when the amount of the co-crosslinking agent is smaller than 19 parts by weight, it is required to increase the amount of the organic peroxide in order to obtain the desired hardness, and high rebound characteristics of the resulting golf ball are not obtained.

[0041] Examples of the organic peroxides, which act as a crosslinking agent or curing agent, include, for example, dicumyl peroxide, 1,1-bis (t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy) hexane, di-t-butyl peroxide, 2,2-di(t-butylperoxy)butane, t-butyl perbenzoate and the like. The preferred organic peroxide is dicumyl peroxide. The amount of the organic peroxide is 0.1 to 3.0 parts by weight, preferably 1.5 to 2.0 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the organic peroxide is smaller than 0.1 parts by weight, the resulting golf ball is too soft, and high rebound characteristics are not obtained. On the other hand, when the amount of the organic peroxide is larger than 3.0 parts by weight, it is required to decrease the amount of the co-crosslinking agent in order to obtain the desired hardness, and high rebound characteristics are not obtained. When these organic peroxides are heated, they decompose to form radicals, which increase the degree of crosslinking between the aforedescribed co-crosslinking agents and base rubber, and enhance the rebound characteristics.

[0042] Examples of the fillers include inorganic fillers such as zinc oxide, silicon oxide, barium sulfate, calcium carbonate, aluminum silicate and mixtures thereof. The inorganic fillers are used as a reinforcing agent, which improves hardness or strength, or a specific gravity (weight) adjuster. However, the preferred filler is zinc oxide, which also functions as a vulcanization aid. The amount of the filler is 10 to 60 parts by weight, preferable 12 to 50 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the filler is larger than 60 parts by weight, the weight ratio of the rubber component in the resulting golf ball is small, and the rebound characteristics are degraded. On the other hand, when the amount of the filler is smaller than 10 parts by weight, it is difficult to adjust the weight of the resulting golf ball, or the durability is degraded.

[0043] In the golf ball of the present invention, it is required for the vulcanization stabilizer to be selected from the group consisting of hydroquinone and derivatives thereof. Examples of the vulcanization stabilizers include 2,5-di-t-butylhydroquinone, 2,5-di-t-amylhydroquinone, 2,6-dimethylhydroquinone, bromohydroquinone, 2,3,5,6-tetrachlorohydroquinone and the like. The preferred vulcanization stabilizer is 2,5-di-t-butylhydroquinone, in view of low toxicity, general-purpose properties and proper stability of free radical. It is required for the amount of the vulcanization stabilizer to be within the range of 0.05 to 2.0 parts by weight, preferably 0.1 to 1.0 parts by weight, more preferably 0.1 to 0.5 parts by weight, based on 100 parts by weight of the base rubber. When the amount is smaller than 0.05 parts by weight, technical effects accomplished by the presence of the vulcanization stabilizer is not sufficiently obtained, and the rebound characteristics are degraded. On the other hand, when the amount is larger than 2.0 parts by weight, the amount of the vulcanization stabilizer, which also acts as a vulcanization initiator, is too large, and the resulting golf ball is brittle, which degrades the durability.

[0044] Where appropriate, it is possible to compound a component which is typically used in the manufacture of one-piece solid golf ball together with the rubber composition for the one-piece solid golf ball of the present invention; pigments such as titanium dioxide and the like, and other additives such as antioxidants or peptizing agents, softening agents and the like.

[0045] The one-piece solid golf ball of the present invention can be obtained by mixing the rubber composition with a proper mixer such as a mixing roll, kneader and the like, and then press-molding and vulcanizing the mixture under applied heat in a mold. The vulcanizing, of which the condition is not limited, is typically conducted at 130 to 240° C. and 2.9 to 11.8 MPa for 15 to 60 minutes. The vulcanizing may be conducted in two or more stages of the temperature.

[0046] At the time of molding the golf ball, many depressions called “dimples” may be optionally formed on the surface of the golf ball. Furthermore, paint finishing or marking with a stamp may be optionally provided for commercial purposes after the golf ball is molded.

[0047] In the one-piece solid golf ball of the present invention, it is required for the golf ball to have a center hardness in JIS-C hardness of 55 to 68, preferably 58 to 63, more preferably 60 to 63. When the center hardness is smaller than 55, the durability of the resulting golf ball is degraded. In addition, the golf ball is too soft at the time of hitting, and the shot feel is poor so that it is felt the rebound characteristics are degraded. On the other hand, when the center hardness is larger than 68, the shot feel is poor so that it is felt the center portion of the ball is hard.

[0048] In the one-piece solid golf ball of the present invention, it is required for the golf ball to have a surface hardness in JIS-C hardness of 75 to 90, preferably 80 to 90, more preferably 80 to 87. When the surface hardness is smaller than 75, the shot feel is heavy and poor. In addition, the rebound characteristics are degraded, which reduces the flight distance. On the other hand, when the surface hardness is larger than 90, the golf ball is too hard, and the shot feel is poor. The term “center hardness” as used herein refers to the hardness, which is determined by cutting the resulting golf ball into two equal parts and then measuring a JIS-C hardness at its center point in section. The term “surface hardness” as used herein refers to the hardness, which is determined by measuring a JIS-C hardness at the surface of the resulting golf ball.

[0049] In the one-piece solid golf ball of the present invention, it is required for the center hardness to be the minimum hardness in the golf ball, and it is required for a difference between the maximum hardness and minimum hardness in the golf ball to be within the range of 16 to 25, preferably 20 to 25, more preferably 22 to 25. When the hardness difference is smaller than 16, the shot feel is heavy and poor and the flight performance is degraded. On the other hand, when the hardness difference is larger than 25, the surface portion of the golf ball is only too hard and the golf ball is brittle, and the durability is degraded.

[0050] Generally, polybutadiene rubber having large Mw, such as the polybutadiene rubber (a) is used in larger amount in order to improve the rebound characteristics. Thereby, the difference between the center hardness and surface hardness is large, and the shot feel is improved, but the durability is degraded with using only the polybutadiene rubber (a). The one-piece solid golf ball having excellent rebound characteristics (flight performance), excellent durability and excellent processability is obtained by using a combination of the polybutadiene (a) and polybutadiene (b). In addition, the shot feel is good by controlling the difference between the maximum hardness and minimum hardness in the golf ball to a specified range.

[0051] Generally, in golf balls having the same compression (deformation amount), when a difference between the maximum hardness and minimum hardness in the golf ball is small, that is, the golf ball has even hardness distribution, the rebound characteristics are degraded. However, the hardness distribution is even, the shot feel is poor. On the other hand, the hardness difference is large, the shot feel is good. In Japanese Patent Kokai Publication No. 177973/1990 described above, even hardness distribution, that is, small hardness difference is accomplished by adding a vulcanization stabilizer to rubber composition for the golf ball. The resulting golf ball thereby has excellent rebound characteristics, but has poor shot feel. On the other hand, in the golf ball of the present invention, golf ball having high rebound characteristics and large hardness difference can be obtained by adding hydroquinone or derivatives thereof to rubber composition for the golf ball and vulcanizing the rubber composition at high temperature to control reaction rate. The golf ball of the present invention has high surface hardness when compared with a golf ball having the same compression (deformation amount). Therefore it is difficult to cut the surface of the golf ball of the present invention, and the golf ball has excellent cut resistance.

[0052] In the solid golf ball of the present invention, the golf ball has a deformation amount (compression) when applying from an initial load of 98 N to a final load of 1275 N of 2.0 to 4.0 mm, preferably 2.0 to 3.0 mm. When the deformation amount is smaller than 2.0 mm, the resulting golf ball is too hard, and shot feel is poor. On the other hand, when the deformation amount is larger than 4.0 mm, the resulting golf ball is too soft, and the rebound characteristics are degraded.

[0053] The weight of the golf ball is limited to the range of not more than 45.92 g in accordance with the regulations for large size golf balls, but the lower limit is not limited. The one-piece solid golf ball of the present invention has a weight of 44.0 to 45.8 g, preferably 44.2 to 45.8 g. When the weight of the golf ball is smaller than 44.0 g, the golf ball loses inertia on a flight, and stalls, which reduces the flight distance. On the other hand, when the weight of the golf ball is larger than 45.8 g, the shot feel is heavy and poor.

[0054] The diameter of the one-piece golf ball of the present invention can be within the range of 41.0 to 44.0 mm, but it is preferably within the range of not less than 42.67 mm in accordance with the regulations for large size golf balls, typically it is about 42.75 mm.

EXAMPLES

[0055] The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope of the present invention.

[0056] (Examples 1 to 3 and Comparative Examples 1 to 6)

[0057] The rubber compositions having the formulation shown in Tables 1 and 2 were mixed, and the mixtures were then press molded at the vulcanization conditions shown in the same Tables in a mold, which is composed of an upper mold and a lower mold having a hemispherical cavity, to obtain one-piece golf balls having a diameter 42.75 mm. The Mooney viscosity, Mw, Mn and content of cis-1,4-butadiene of the polybutadiene rubbers used are shown in Table 3. TABLE 1 (parts by weight) Comparative Example No. Example No. Composition 1 2 3 1 2 Polybutadiene A 90 90 80 100 50 Polybutadiene B 10 10 20 — — Polybutadiene C — — — — 50 Zinc oxide 23 23 23 23 23 Methacrylic acid 24 24 24 24 24 Vulcanization 0.15 0.50 0.20 0.20 0.20 stabilizer *1 Dicumyl peroxide 1.40 2.40 1.40 1.60 1.45 Vulcanization condition The first Temp. (° C.) 171 171 171 165 160 stage Time (min) 17 17 17 20 25 The sec- Temp. (° C.) — — — — 170 ond stage Time (min) — — — — 5

[0058] TABLE 2 (parts by weight) Comparative Example No. Composition 3 4 5 6 Polybutadiene A 90 90 90 100 Polybutadiene B 10 10 10 — Polybutadiene C — — — — Zinc oxide 23 23 23 23 Methacrylic acid 24 24 24 24 Vulcanization stabilizer *1 — 2.20 0.20 0.20 dicumyl peroxide 0.65 9.60 1.50 1.60 Vulcanization condition The first Temp. (° C.) 171 171 165 171 stage Time (min) 17 17 23 17 The second Temp. (° C.) — — — — stage Time (min) — — — —

[0059] TABLE 3 Polybutadiene A B C Trade name BR18 BR230 BR150L Manufacturer JSR Co. Ube Ube Industries Industries Catalyst Nickel Cobalt Cobalt Mooney viscosity 60 38 43 [ML₁₊₄(100° C.)] *1 Content of cis-1,4- 96 98 98 polybutadiene (%) *2 Weight average molecular 100 × 10⁴  63 × 10⁴ 56 × 10⁴ weight (Mw) *3 Number average molecular 23 × 10⁴ 16 × 10⁴ 24 × 10⁴ weight (Mn) *3 Ratio (Mw/Mn)   4.3   3.9   2.3

[0060] With respect to the resulting one-piece solid golf ball, the compression, coefficient of restitution durability, hardness distribution including center hardness and surface hardness in JIS-C hardness and shot feel were measured or evaluated, and the results are shown in Tables 4 and 5. The difference between the maximum hardness and minimum harness in the golf ball was calculated, and the results (hardness difference) are shown in the same Tables. The test method are as follows.

[0061] Test Method

[0062] (1) Compression (Deformation Amount)

[0063] The compression was determined by measuring the deformation amount when applying from an initial load of 98 N to a final load of 1275 N on the golf ball. The compression is indicated by an index when an inverse number of the deformation amount of Comparative example 1 is 100. The higher the index is, the higher the hardness is.

[0064] (2) Coefficient of Restitution

[0065] A metal cylindrical article having a weight of 198.4 g was struck at a speed of 40 cm/sec against a golf ball, which is in a stationary state, and the velocity of the cylindrical article and the golf ball before and after the strike were measured. The coefficient of restitution of the golf ball was calculated from the velocity and the weight of both the cylindrical article and the golf ball. The measurements were conducted five times for each golf ball, with the mean value being taken as the coefficient of restitution of each ball and indicated by an index when that of Comparative Example 1 is 100. A higher index corresponded to a higher rebound characteristic, and thus a good result.

[0066] (3) Durability

[0067] The same impact force was repeatedly applied to a golf ball, that is, a golf ball was repeatedly hit at a head speed of 45 m/second. The durability is determined by measuring the number of strike until the surface of the golf ball cracks, and is indicated by an index when that of Comparative Example 1 is 100. The higher the index is, the more excellent the durability is.

[0068] (4) Durability (of Golf Ball Having Nicks)

[0069] The durability was determined as described in the (3) except for cutting two nicks on a golf ball in advance. The nicks are positioned on the parting line and apex of the golf ball and have a depth of 2 mm. The result is indicated by an index when that of Comparative Example 1 is 100 as described in the (3).

[0070] (5) Hardness and Hardness Distribution

[0071] The hardness and hardness distribution is determined by cutting a golf ball into two equal parts and then measuring a JIS-C hardness at a center point, at a distance of 5 mm, 10 mm and 15 mm from the center point, and at a surface in section. The JIS-C hardness was measured at 20° C. using a JIS-C hardness meter according to JIS K 6301.

[0072] (6) Shot Feel

[0073] The shot feel of the golf ball is evaluated by 10 top amateur golfers on a maximum scale of 10 points according to a practical hitting test using a driver having metal head. The higher the score is, the better the shot feel is. The evaluation criteria are as follows.

[0074] ∘: Total score of not less than 80 points

[0075] Δ: Total score of not less than 60 points and less than 80 points

[0076] x: Total score of less than 60 points

[0077] Test Results TABLE 4 Comparative Example No. Example No. Core composition 1 2 3 1 2 compression 100 100  99 100 100 Coefficient of restitution 102 102 101 100  99 Durability 100 100 100 100 100 Durability (nicks) 180 175 250 100  80 Hardness distribution (JIS-C) Center point  60  61  64  66  68  5 mm from the center point  68  67  69  68  71 10 mm from the center point  73  75  74  74  75 15 mm from the center point  80  81  79  76  75 Surface  85  85  82  76  75 Hardness difference  25  24  18  10  7 Shot feel ∘ ∘ ∘ Δ Δ

[0078] TABLE 5 Comparative Example No. 3 4 5 6 Core composition compression 108 100 100 101 Coefficient of restitution 97 101 98 102 Durability 100 60 100 70 Durability (nicks) 180 40 120 90 Hardness distribution (JIS-C) Center point 65 62 65 60 5 mm from the center point 68 68 67 68 10 mm from the center point 73 73 70 73 15 mm from the center point 77 76 73 80 Surface 82 82 75 85 Hardness difference 17 20 10 25 Shot feel Δ Δ Δ ∘

[0079] As is apparent from the results of Tables 4 and 5, the one-piece solid golf balls of the present invention of Examples 1 to 3 as compared with the golf balls of Comparative Examples 1 to 6 have excellent rebound characteristics, excellent durability and good shot feel.

[0080] On the other hand, in the golf balls of Comparative Examples 1 and 2, which corresponds to the golf balls described in Japanese Patent Kokai Publication No. 177973/1990 and Japanese Patent No. 2644226, the coefficient of restitution is equal to or slightly lower than that of the golf balls of Examples, but a difference between the maximum hardness and minimum hardness in the golf ball (hardness difference) is small, and the shot feel is poor. In the both golf balls, the surface hardness is low, and it is easy to cut on the surface thereof and the durability of the golf ball having nicks is very poor. In the golf ball of Comparative Example 1, only polybutadiene having high Mooney viscosity (polybutadiene (a)) was used, and the durability was poor. In the golf ball of Comparative Example 2, the amount of polybutadiene having low Mooney viscosity (polybutadiene (b)) is large, and the coefficient of restitution is low.

[0081] In the golf ball of Comparative Example 3, the vulcanization stabilizer was not used, and the coefficient of restitution is low. In the golf ball of Comparative Example 4, the amount of the vulcanization stabilizer was large, and it is required to compound a large amount of organic peroxide together with the rubber composition for the golf ball in order to adjust the compression to a proper range.

[0082] In the golf ball of Comparative Example 5, which used the same rubber composition as that of Example 1 and different vulcanization condition from that of Example 1, the hardness difference is small, and the shot feel is poor. In addition, the surface hardness is low, and it is easy to cut the surface of the golf ball and the durability of the golf ball having nicks is very poor. In the golf ball of Comparative Example 6, the hardness difference is within the range of the present invention, but only polybutadiene having high Mooney viscosity (polybutadiene (a)) was used, and the durability was poor. 

What is claimed is:
 1. A one-piece solid golf ball formed from a rubber composition comprising a base rubber, a co-crosslinking agent, an organic peroxide, an inorganic filler and a vulcanization stabilizer, and the base rubber comprises a polybutadiene mixture consisting of (i) a polybutadiene (a) containing a cis-1,4 bond of not less than 96% and having a Mooney viscosity of 50 to 85 ML₁₊₄(100° C.) and a weight average molecular weight (Mw) of 70×10⁴ to 120×10⁴, synthesized using nickel-containing catalyst, and (ii) a polybutadiene (b) containing a cis-1,4 bond of not less than 96% and having a Mooney viscosity of 30 to 50 ML₁₊₄(100° C.) and a weight average molecular weight (Mw) of not less than 55×10⁴ and less than 70×10⁴, synthesized using cobalt-containing catalyst, a weight ratio (a)/(b) being 95/5 to 70/30, wherein the vulcanization stabilizer is selected from the group consisting of hydroquinone and derivatives thereof, and an amount of the vulcanization stabilizer is 0.05 to 20 parts by weight, based on 100 parts by weight of the base rubber, and the golf ball has a center hardness in JIS-C hardness of 55 to 68 and a surface hardness in JIS-C hardness of 75 to 90, the center hardness is the minimum hardness in the golf ball, and a difference between the maximum hardness and minimum hardness in the golf ball is within the range of 16 to
 25. 2. The one-piece solid golf ball according to claim 1, wherein the vulcanization stabilizer is 2,5-di-t-butylhydroquinone.
 3. The one-piece solid golf ball according to claim 1, wherein the polybutadiene (b) synthesized using cobalt-containing catalyst has a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 3.0 to 5.0 